Field of the Invention
The present invention relates to a radio-frequency input stage having a radio-frequency coupling element to which an input signal can be supplied and which is coupled to a first and to a second tunable radio-frequency resonant circuit, which are connected to one another on the input side.
Radio-frequency input stages (front ends) with tunable radio-frequency resonant circuits which are connected in parallel with one another and are configured for different frequency bands are normally provided in two-band and three-band TV tuners.
A radio-frequency input circuit of this generic type is specified in the document Published, European Patent Application EP 0 299 578 A1, corresponding to U.S. Pat. No. 4,851,796.
There, a radio-frequency coupling element is connected on one side to an antenna and on the other side to in each case one first, one second and one third radio-frequency resonant circuit. The coupling element is in this case formed from an xcex7 section with a series capacitance and two shunt paths.
It is accordingly an object of the invention to provide a radio-frequency input stage that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has a simple circuit configuration, is improved with regard to the mutual interference between the individual bands, and can be produced at low cost.
With the foregoing and other objects in view there is provided, in accordance with the invention, a radio-frequency input stage. The input stage contains a first tunable radio-frequency resonant circuit having an input, a second tunable radio-frequency resonant circuit having an input connected to the input of the first tunable radio-frequency resonant circuit, and a radio-frequency coupling element for receiving an input signal and coupled to the first and second tunable radio-frequency resonant circuits. The radio-frequency coupling element contains a reference-ground potential terminal and a capacitive element having a first terminal coupled to the first and second tunable radio-frequency resonant circuits and a second terminal connected to the reference-ground potential terminal.
According to the invention, the object is achieved by a radio-frequency input stage of the type mentioned initially in which the coupling element has a capacitive element which is coupled on one side to the first and second radio-frequency resonant circuits, and is coupled on the other side to the reference-ground potential connection.
The radio-frequency input stage does not require any changeover switches between the individual tuning circuits. Each radio-frequency resonant circuit can be configured for its own frequency band. The individual frequency bands may be adjacent. The capacitive element, which forms a low-pass filter, effectively suppresses the undesirable influences in particular of the low and possibly central frequency band on the high frequency band.
The splitting of a radio-frequency received signal (which, for example, is input at an antenna) into the various frequency bands and between the various resonant circuits is carried out by a simple parallel circuit. No switching diodes are required for this purpose. The described configuration can be produced in a simple and low-cost manner and is improved with regard to the suppression of undesirable influences on bands that have in each case not been selected.
In one advantageous embodiment of the present invention, the radio-frequency coupling element has a high-pass filter. For television or radio reception, for example, the desired frequency band or the desired frequency bands is or are not close to 0 Hertz, so that undesirable frequencies which are below the desired frequency bands can be suppressed by a high-pass filter.
In a further advantageous embodiment of the present invention, the high-pass filter has an inductance, which is connected on one side to the air interface and to the radio-frequency resonant circuits, and is connected on the other side to the reference-ground potential. This allows a particularly simple implementation of a high-pass filter.
In a further advantageous embodiment of the present invention, a third, tunable radio-frequency resonant circuit is provided, and is connected in parallel with the first and second radio-frequency resonant circuits. A third, tunable radio-frequency resonant circuit can be used to produce a three-band TV tuner which, for example, can cover a range from 470 to 960 MHz in a first frequency band, a range from 160 to 470 MHz in a second frequency band, and a frequency range from 40 to 160 MHz in a third band.
In a further advantageous embodiment of the present invention, the radio-frequency resonant circuits each have a series inductance on the input side for impedance conversion, one side of which is in each case connected to the coupling element. The impedance conversion is particularly advantageous for matching the impedance (which varies as a result of the tuning of the resonant circuit) of the resonant circuit to that of the air interface.
In a further advantageous embodiment of the present invention, the radio-frequency resonant circuits each have an inductance in parallel with a series circuit formed from a capacitance diode and a capacitance. The resonant circuits may be controllable by a common tuning voltage, by the capacitance diodes each having a variable capacitance.
In a further advantageous embodiment of the present invention, a capacitance for suppressing mirror-image frequencies is in each case connected between an input, which is connected to the coupling element, and an output, which is coupled to an amplifier, of the radio-frequency resonant circuits. It may be desirable to suppress mirror-image frequencies when, for example, mixers are provided for conversion of an amplified output signal from the resonant circuits to an intermediate-frequency signal.
In a further advantageous embodiment of the present invention, a capacitance for widening the bandwidth of the respective resonant circuit is connected between the series inductance and an output, which is coupled to an amplifier, of the radio-frequency resonant circuits.
In a further advantageous embodiment of the present invention, the coupling element has a first bandstop filter for suppressing undesirable intermediate frequencies.
In a further advantageous embodiment of the present invention, the coupling element has a second bandstop filter for suppressing further undesirable frequencies, and the second bandstop filter is connected upstream by the first bandstop filter. By way of example, the second bandstop filter may be configured such that it suppresses signals in the frequency range of the so-called citizen band (CB). The first and second bandstop filters may, for example, be formed by an inductance which is connected in parallel with a capacitance and may, for example, be connected on the input side to an antenna and on the output side to the inputs of the resonant circuits. The first and second bandstop filters may in this case be connected in series with one another, with the intermediate-frequency bandstop filter advantageously being connected downstream from the second bandstop filter. In particular, in this case, a connecting node, to which a high-pass filter can be connected, may be formed between the bandstop filters.
In a further advantageous embodiment of the present invention, finally, the capacitive element has a capacitance value of 0.5 picofarads and the radio-frequency input stage has an overall tuning range from 40 to 870 MHz.
The first radio-frequency resonant circuit may in this case be configured for a high frequency band, the second radio-frequency resonant circuit for a central frequency band, and the third radio-frequency resonant circuit for a low frequency band. The capacitive element which forms the low-pass filter may advantageously be disposed closer to the second radio-frequency resonant circuit than to the first radio-frequency resonant circuit in a practical circuit implementation, so that this results in a shorter connection to the former than to the latter, in which case the coupling element may be connected closer to the input of the first radio-frequency resonant circuit than to the input of the second radio-frequency resonant circuit.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a radio-frequency input stage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.